1. Field of the Invention
The present invention relates to a branching filter that uses an surface acoustic wave (SAW) resonance type filter used for compact mobile communication equipment for portable telephones and the like.
2. Description of Related Art
In recent years, advances have been made in the development of terminals for compact, light mobile communication equipment such as portable telephones. RF (Radio Frequency) filters are incorporated into these terminals. Surface acoustic wave (SAW) resonance type filters are used for this RF filter.
Accompanying the development of these terminals is a demand for the parts to be made more compact and to have higher performance. Therefore, there is also a demand for more compact, higher performance SAW resonance filters (also called SAW elements).
FIG. 9 is a block diagram of a structural example of a conventional portable telephone branch filter.
The branch filter 10 shown in FIG. 9 comprises an antenna (ANT) terminal 11, an LC chip 12, a transmission filter 13, an Rx-branching filter circuit strip line 14, a receiving filter 15, a transmission (Tx) terminal 16, and a receiving (Rx) terminal 17. The LC chip 12 is provided between the ANT terminal 11 and ground. The transmission filter 13 is connected between ANT terminal 11 and Tx terminal 16, and the series circuit of the Rx-branching filter circuit strip line 14 and the receiving filter 15 are connected in this sequence between the ANT terminal 11 and the receiving terminal 17.
FIG. 10 is a circuit structural figure of the specific circuit structure of the branching filter shown in FIG. 9. Reference number 11 is the ANT terminal, 2, 3, and 4 are branching filter circuit strip lines (inductance) (equivalent to 14 in FIG. 9), 13 is the transmission filter, 15 is the receiving filter, 16 is the transmission (Tx) terminal, and 17 is the receiving (Rx) terminal.
Conventionally, with this type of portable telephone branching filter, the transmission and receiving filters were each composed using dielectric resonators.
FIGS. 11 and 12 show a portable telephone branching filter and mounting aspect, respectively. FIG. 11 is a schematic perspective view of the front surface, and FIG. 12 is a schematic perspective view of the back surface.
As is clear from the structural examples shown in FIGS. 11 and 12, chips 13 and 15 of the transmission and receiving filters are incorporated into on-board substrate 9. Branching filter circuit strip lines 2, 3, and 4 are provided as structural elements on this on-board substrate 9. As is also clear from FIG. 12, this on-board substrate 9 comprises an insulation substrate 9a such as a resin substrate, low temperature sinter substrate, or aluminum substrate, a metallized conductive coating pattern 9b provided thereon, and an insulation pattern 9c formed by exposing substrate 9a. Branching filter circuit strip lines 2, 3, and 4 are formed in continuum with conductive coating pattern 9b. 
With this type of portable telephone branching filter, each chip of the transmission filter 13 and the receiving filter 15 is provided divided on separate piezoelectric substrates. Then, these two piezoelectric substrates are individually incorporated into one on-board substrate 9, offering the merit of excellent insulation characteristics for both filters.
However, besides the piezoelectric substrate on which are provided the chips of these transmission and receiving filters 13 and 15, an on-board substrate 9 with a space for incorporating the Rx-branching filter circuit strip line 14 and the LC chip 12 (thus, branching filter circuit strip lines 2, 3, and 4 shown in FIG. 11) is needed, so the on-board substrate becomes large, and the connecting wiring for the branching filter structure becomes long. Because of this, the structure of this on-board substrate 9 becomes complex, and the area occupied by the connecting wiring increases. This inhibits making the on-board substrate and thus the branching filter more compact.
On the other hand, a branching filter has been developed that uses a SAW resonator for the transmission filter and receiving filter (Japanese Patent Laid-open No. 6-97761). For the branching filter that uses a SAW resonance type filter disclosed in this publication, the transmission and receiving filters comprise ladder-type resonator filters with a structure similar to a serial arm SAW resonator and a parallel arm SAW resonator. With this conventional branching filter, it is possible to make the branching filter more compact to some degree, but the problem of insulation between filters has not been looked into yet. FIG. 13 is a schematic perspective view showing a structural example of the branching filter disclosed in this Japanese Patent Laid-open No. 6-97761. This branching filter has a structure with which the structural elements are incorporated into package 20. Specifically, inside a package construction 21A is provided a ground layer 21B, an impedance matching element 22, a phase adjustment element 23, a trap circuit 24, a transmission SAW filter element 25, and a receiving SAW filter element 26.
In this way, the conventional branching filter disclosed in Japanese Patent Laid-open No. 6-97761 is structured to house in a single package 20 the transmission SAW filter element 25, the receiving SAW filter element 26, an LC chip, and an Rx-branching filter circuit strip line.
However, in this case, particularly because the transmission SAW filter element 25, the receiving SAW filter element 26, the LC chip (phase adjustment element 23), and the Rx-branching filter circuit strip line (impedance matching element 22) are housed within the same package 20, there are problems including a degradation of the insulation characteristics between the transmission area and receiving area, and a degradation of the branching filter characteristics as an interaction works between the connecting wires.